Transparent exterior partitions of aircraft must be durable enough to secure operators and equipment from harsh environmental conditions. In addition, to enable operation of sensitive equipment employed in reconnaissance, such as is used for infrared imaging systems and for weapon delivery applications, these materials must be transmissive to light in a spectrum ranging from visible to infrared wavelenths.
Existing exterior partitions include barriers, such as large multispectral windows. Such windows often limit system design in FLIR (forward looking infrared) and infrared imaging systems. The inability of these windows to withstand rain abrasion has been a continuing problem for many years with no satisfactory solution still in sight.
Aircraft typically impact raindrops at high speed, necessitating an external barrier which is sufficiently strong to resist fracture. Raindrops also create minor indentations on the outer surface of conventional windows, imparting distortions to the optical quality of the partition and deleteriously affecting operators vision and the performance of equipment. In addition to strength and hardness, these materials must be resistant to rain or particle erosion caused by extended exposure in use of aircraft over long periods of time.
Thus, the materials of construction must be hard enough to withstand the abrasive effect of raindrops impacting transparent surfaces at high speeds. Further, increased hardness and strength of known materials must be accomplished without diminishing optical transmissivity.
A solution to problems of strength and hardness of optically transmissive barrier is formulation of materials which provide such a compromise of mechanical properites without deleteriously affecting optical quality. Equipment and operating systems housed within aircraft are constricted by the limitation of transparent partitions in the fuselage. Windows constructed of such materials must be formed so as to accommodate particular use requirements.
Attempts to increase the hardness and strength of known materials have included introduction of dopants to a coating material during chemical vapor deposition of that material onto a window or panel substrate. Common window substrates include glass, germanium (Ge), silicon (Si), zinc sulfide (ZnS), and zinc selenide (ZnSe). Dopants added have included arsenic, aluminum, and mixtures thereof. See, for example, Hardened CVD Zinc Selenide for FLIR Windows, Raytheon Company, Research Division, AFML-TR-75-142, dated September 1975. These efforts have all resulted in unacceptable loss of optical transmissivity (See pages 70-71 of the referenced research report).
A need exists, therefor, to preserve the high resolution capability of sophisticated reconnaissance and weapon delivery systems by increasing hardness, strength and durability of known protective window materials without diminishing their optical transmissivity.